Low-frequency loud speaker

ABSTRACT

A low-frequency loudspeaker overcoming a tendency to provide excessive midrange acoustic output. A circular attenuator disk of a low-density material is secured axially to and covers the central radiating surface area of the loudspeaker&#39;&#39;s conical diaphragm, being attached adhesively at its periphery to the diaphragm at a circle spaced inwardly from the outer periphery of the diaphragm. The disk is also attached at its center to the support dome for the loudspeaker, and it lies substantially in a single plane, thereby forming an airtight chamber between the disk and the diaphragm. As a result, the excessive midrange acoustic output efficiency is dissipated within the chamber, only the excessive midrange acoustic output efficiency being attenuated, and this is done without altering the electromagnetic circuit properties of the loudspeaker. The loudspeaker&#39;&#39;s acoustic output performance is thereby brought to a substantially uniform level over its entire operating range.

United States Patent [191 Primary ExaminerStephen J. ,Tomsky I Attorney, Agent, or Firm-Owen, Wickersham & Erickson a r Thomasen Sept. 3, 1974 LOW-FREQUENCY LOUD SPEAKER [57] ABSTRACT [76] Inventor: Norman Leonard Thomasen, 760 A low-frequency loudspeaker overcoming a tendency M k S S i 315 S to provide excessive midrange acoustic output. A cir- Francisco, Calif 94102 cular attenuator disk of a low-density material is secured axially to and covers the central radiating sur- 2 Flled: 1974 face area of the loudspeakers conical diaphragm, [21] Appl. No.: 453,512 being attached adhesively at its periphery to the diaphragm at a circle spaced inwardly from the outer pev riphery of the diaphragm. The disk is also attached at [52] US. Cl. 181/32 R, l79/l8l R center to the Support dome for the loudspeaker, [51] Int. Cl.f H04r 7/00, GlOk 13/(23 and it lies bstantially in a single plane, thereby form- [58] Fleld 0 Search 1 g 1 ing an airtight chamber between the disk and the dial v phragm. As a result, the excessive midrange acoustic output efficiency is dissipated within the chamber, [5 6] References Cited only the excessive midrange acoustic output efficiency UNITED STATES PATENTS being attenuated, and this is done without altering the 2,997,549 8/ 1961 Hassan 181/32 R electro-magnetic circuit properties of the loudspeaker. 1 v 965 Broadley....., 181/3 R The loudspeakers acoustic output performance is 3,213,209 10/1965 Doelitzschm 181/32 R thereby brought to a substantially uniform level over 3,329,777 7/1967 Kliewer l79/l 15.5 R i entire Operating range j 3,555,206 l/197l Dvorsky 181/32 R 3 Claims, 4 Drawing Figures PAIENIEDSEP- 31914 3.833.085

I3 l4 2 II IO l8 ,/ll

EFFECT OF EQUALIZATION ON OVERALL RESPONSE 'EEEEE Iil-fi'fif? a g I FIG 3 S I I .A

V/ l-- LOSS Loss D O. I- l I I l o INCORRECT POW RELATIONSHIP (UNCONTROLLED WOOFER RESPONSE) 4OHZ 2.6KHZ I5KHZ FREQUENCY LIEEEEILNG I DISISDIPATID v v M RAN E A I I FIG 4 CD e L. I B

l IY I I- J I I I g CORRECTED POwER RELATIONSHIP (CONTROLLED WOOFER RESPONSE) 4OHZ 2.6KHZ IS-KHZ FREQUENCY LOW-FREQUENCY LOUD SPEAKER BACKGROUND OF THE INVENTION This invention relates to low-frequency loudspeak- 5 ers.

Many loudspeaker systems employ two types of loudspeakers,one to reproduce the lower audio frequencies of sound while the other is used to reproduce the higher audio frequencies of sound. Such a system is commonly known as a two-way loudspeaker system, and in them a crossover network is commonly used to divide the audible range of sound, so that the lowfrequency loudspeaker handles only the lower octaves of sound and the high-frequency loudspeaker handles ment in any high quality low-frequency loudspeaker is that its cone movement respond instantly at the commencement of an audio signal impulse and, just as quickly, come to a position of' complete rest at the conclusion of that signal impulse. Therefore, the requirement for good transient response in a speaker system calls for a low-frequency loudspeaker whose relatively strong electro-magnetic circuit properties identifies it as being of the high efficiency type. In general, the stronger the loudspeakers magnetic flux density, the better its transient response.

Unfortunately, the basic necessity for such strong magnetic properties has its greatest influence upon a low-frequency loudspeakers acoustic output efficiency over the midrange frequencies. Therefore, a high efficiency loudspeaker-although of the type required by a high quality speaker system-produces a louder level of acoustic output in the midrange frequencies than in the lower frequencies. This occurs because a lowfrequency loudspeakers efficiency is specified in the midrange frequencies, and therefore loudspeaker designersoften refer to it as the loudspeaker s sensitivity region.

In addition to this having greater acoustic output efficiency in the midrange frequencies, another unfortunate drawback of prior-art low-range loudspeakers arises from the fact that the same strong electromagnetic properties which give a high efficiency loudspeaker good transient response and cause a predomi-,

frequency loudspeaker which is to cover the lower fre- 65 quency assignment within a bandwidth of approximately 30 Hz to 3,000 Hz in a two-way system, and at the same time is to provide good transient response, has

two drawbacks: (l) a predominant acoustic output level over the midrange frequencies of its assigned range and (2) an inadequate acoustic output level over the lower frequencies of its assigned range.

The choice of a desired listening level is governed largely by one s keener sensitivity to a speaker systems prevailing loudness in the midrange frequencies of sound. The reason for this fact is that the human hearing process is endowed by nature with greater sensitivity to midrange frequencies than it is to either low or high frequencies. Since ones choice of a listening level is determined from a natural, and yet unconscious reference to the midrange frequencies of sound, it follows that when a loudspeakers midrange acoustic output is at a level of, for example, 6 decibels higher than its lowfrequency output level, then the low frequencies are not heard at an adequate loudness level with respect to the inherent uniformity present in todays recorded musical source material and in todays related amplification equipment.

One object of the present invention is to overcome the undesirable predominant midrange acoustic output efficiency of a lowfrequency loudspeaker in these instances where such predominance in output is due to a loudspeakers essential requirement that it incorporate an electromagnetic coupling factor high enough in value to achieve good transient response. This object is accomplished by'retaining in the loudspeaker a high value of electromagnetic coupling in its magnetic airgap for the purpose of obtaining good transient response while simultaneously lowering its acoustic output efficiency in the midrange only. This can be done to a degree which causes the loudspeaker to produce substantially the same level of acoustic output over the midrange frequencies as it produces over the lowest frequencies of audible sound. Note that only the loudspeakers excessive midrange acoustic output level is lowered,not the electromagnetic properties which it needs to obtain good transient response. To this end the basic construction of a'high efficiency type loudspeaker itself is not altered, but modified.

SUMMARY OF THE INVENTION This invention provides a low-frequency loudspeaker having high efficiency and yet overcoming the tendency to provide excessive midrange acoustic output. Its main support structure supports a voice coil. A supporting dome connected to the coil supports the inner periphery of a main conical diaphragm, the outer periphery being supported by the main support structure.

A circular attenuator disk of a low-density material is secured axially onto and covers the central radiating surface area of the diaphragm. The disk is attached adhesively at its periphery to the diaphragm at a circle spaced inwardly from the diaphragms outer periphery. The center of the disk is attached to the dome and the disk lies substantially in a single plane, and it forms an airtight chamber between the disk and the diaphragm.

As a result, the excessive midrange acoustic output efficiency is dissipated within the airtight chamber, only the excessive midrange acoustic output efficiency of the loudspeaker being attenuated. This is done without altering the electromagnetic circuit properties of the loudspeaker which are responsible for its tendency toward excessive midrange output efficiency. Thereby the invention equalizes the loudspeakers acoustic output performance to a substantially uniform level over its entire operating range.

Other objects and advantages of the invention will appear from the following description of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

' FIG. 1 is a simplified view in front elevation of a lowfrequency range loudspeaker embodying the principles of the invention.

FIG. 2 is a view in section taken along the line 22 in FIG. 1.

FIG. 3 is a graphical diagram plotting output in decibels against frequencies in Hertz and kiloHertz for the uncorrected loudspeaker.

FIG. 4 is a view like FIG. 3 for the corrected loudspeaker of this invention, differing from the uncorrected loudspeaker only in inclusion of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT FIGS. 1 and 2 show the moving system of a lowfrequency loudspeaker 10. A main cone or conical diaphragm 11 is attached to a voice coil structure 12 and, at its outer line 13 to a support cone 14 all conventional. However, in the present invention a circular attenuator disk 15 is positioned centrally with respect to and secured onto the main c one 11. This attenuator disk 15 is preferably of closed-cell neoprene foam; typically, it may have a density in the order of to 12 lbs. per cubic foot and a uniform thickness of one-eighth inch i001 5 inch. The disk preferably covers about 40 percent of the effective radiating surface of the main cone 11. For example, for a main cone 1 1 having an effective radiating surface diameter of 7- /2 inches, the disk 15 may have a diameter of 4-% inches. The disk 15 may be secured to the main cone 11 by a suitable adhesive cement applied at the circular perimeter 16 of the disk 15. The disk 15 is also secured at its center by applying a film of suitable adhesive cement to a central area 17 of a supporting dome 18, which is preferably of the usual fiber type. The dome 18 is of such suitable size that its most forward centrally exposed surface area 17 lies in substantially the same plane as the circular perimeter 16 of the disk 15, where it intersects with the main cone 11. In this manner, the disk 15 is rigidly supported to prevent flexure at its two supporting areas 16 and 17. Its adhesive bond with the main cone 11 forms an airtight chamber 20 between the disk 15 and the main cone 11.

The structure dissipates the excessive and predominant midrange acoustic output of the loudspeaker 10 within the enclosed chamber 20 formed and enveloped by the disk 15 and the main cone 11. The degree of attenuation necessary to reduce the excessive acoustic output (which is found to emanate from that portion of the main cone 11 which is covered by the disk 15) is governed by the density and thickness of the disk 15.

Since the degree of attenuation needed varies between loudspeakers, objective acoustic measurements are recommended as an aid in determining the optimum density and thicknessof the disk 15 for a particular loudspeaker. The degree to which the excessive midrange acoustic output for any given loudspeaker is to be attenuated can be controlled by altering either the density or the thickness of the disk 15 or both.

A major benefit of this invention is that it enables the excessive midrange acoustic output of a low-frequency loudspeaker to be lowered to a level that is substantially equal to its low-frequency output level. It achieves this benefit without diminishing that high order of electromagnetic coupling which gives the loudspeaker its high-efficiency and, therefore, its ability to respond instantly to audio signal transients.

Another benefit of this invention is that by attenuating the excessive midrange output level of the loudspeaker, the crossover transition between the lowfrequency loudspeaker and the high-frequency loudspeaker can occur at a higher frequency, where power demands are lower. Such a crossover transition helps to prevent not only distortion from the high-frequency I loudspeaker but also the danger of failure thereof due to power overloads to which it might otherwise be subjected at a lower crossover frequency.

This invention, therefore, obtains desirable lowfrequency acoustic output characteristics previously obtainable only by much more expensive means, such as employing costlier and much larger low-frequency loudspeakers or resorting to three-lor' four-way system designs, which employ greater complexities in their design arrangements, or employing expensive electronic equalizer apparatus for attenuating excessive output. The invention accomplishes its objectives at small cost compared to other methods presently available for controlling the acoustic output characteristics of a loudspeaker.

FIGS. 3 and 4 show in graphical form the results of the present invention. Since the drawings are fully labeled, no further explanation is needed.

The entrapped air in the enclosed space 20 acts as a medium for transmitting motion. However, the motional disposition of the disk 15, because of the disks resiliency, is positive at low frequencies, and as the frequencies rise into the midrange region its motional disposition, except at its supporting areas 16 and 17, becomes negative. This is because of the effects of inertia; that is, the accelerated forces acting upon the disk 15 in the higher frequencies are not transmitted throughout the disk 15, because of the inertia of the material used in the disk. The advantages are significant since under these circumstances, the additional mass of the disk 15 does not effect any significant increase in power requirements from the power amplifier (as occurs where mass loading of the diaphragm is used in the prior art). The disk 15, therefore, becomes a true attenuator of sound where attenuation is called for.

To those skilled in the art to which this invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.

I claim:

1. A low-frequency loudspeaker having a main support structure, a voice coil supported by said main support structure, a supporting dome connected to the coil, and a main conical diaphragm supported by said dome at an inner periphery and by said main support structure at an outer periphery, said loudspeaker having high efficiency and overcoming a tendency to provide excessive midrange acoustic output, characterized by a circular attenuator disk of a low-density material secured axially onto and covering the central radiating surface area of said diaphragm,

said disk being attached adhesively at its periphery to said diaphragm at a circle spaced inwardly from the outer periphery of said diaphragm, said disk being attached at its center to said dome and lying substantially in a single plane,

thereby forming an airtight chamber between said disk and said diaphragm,

wherein said excessive midrange acoustic output efficiency is dissipated within said chamber, only the excessive midrange acoustic output efficiency of the loudspeaker being attenuated, all without altering the electromagnetic circuit properties of the loudspeaker which are responsible for its tendency toward excessive midrange output efficiency, thereby equalizing the loudspeakers acoustic output performance to a substantially uniform level over its entire operating range.

2. The low-frequency loudspeaker of claim 1 wherein said attenuator disk covers approximately 40 percent of the area of said diaphragm.

3. The low-frequencyloudspeaker of claim 1 wherein said attenuator disk is of neoprene foam having a closed cell structure, a density of aboutten to twelve pounds per cubic foot, and a thickness of about oneeighth inch. 

1. A low-frequency loudspeaker having a main support structure, a voice coil supported by said main support structure, a supporting dome connected to the coil, and a main conical diaphragm supported by said dome at an inner periphery and by said main support structure at an outer periphery, said loudspeaker having high efficiency and overcoming a tendency to provide excessive midrange acoustic output, characterized by a circular attenuator disk of a low-density material secured axially onto and covering the central radiating surface area of said diaphragm, said disk being attached adhesively at its periphery to said diaphragm at a circle spaced inwardly from the outer periphery of said diaphragm, said disk being attached at its center to said dome and lying substantially in a single plane, thereby forming an airtight chamber between said disk and said diaphragm, wherein said excessive midrange acoustic output efficiency is dissipated within said chamber, only the excessive midrange acoustic output efficiency of the loudspeaker being attenuated, all without altering the electromagnetic circuit properties of the loudspeaker which are responsible for its tendency toward excessive midrange output efficiency, thereby equalizing the loudspeaker''s acoustic output performance to a substantially uniform level over its entire operating range.
 2. The low-frequency loudspeaker of claim 1 wherein said attenuator disk covers apProximately 40 percent of the area of said diaphragm.
 3. The low-frequency loudspeaker of claim 1 wherein said attenuator disk is of neoprene foam having a closed cell structure, a density of about ten to twelve pounds per cubic foot, and a thickness of about one-eighth inch. 